The present invention relates to microwave varactor circuits, and in particular to microwave voltage-controlled oscillators.
Many varactors are prone to developing a significant resistive component over low-bias portions of their operating range. This resistance can lead to rectification of the RF signal appearing across the varactor. This rectified component will cause a rapid change in the varactor's capacitance, so that the varactor starts to behave like a parametric oscillator. This can lead to severe spectral breakup. This is extremely undesirable in many applications.
That is, it has been discovered that RF rectification in the varactors of a VCO can generate as much as 7 volts across the varactors. This causes a dynamic change in frequency, and the resulting change in power at the new frequency causes a change in the bias level seen by the varactor, which is now equal to the applied DC voltage plus the RF rectified voltage. This again changes the operating frequency of the oscillator. This action causes severe spectral breakup.
The present invention provides a network on the varactor tuning line which can sink the rf rectified current as it is generated. This network can sink the substantial amounts of RF current which can be supplied from the RF rectification (up to 30 mA in a sample application). Moreover, the current supplied by the network when operating near the breakdown voltage of the varactor is limited to less than 1 mA, to protect the varactor. Moreover, the regulated bias voltage is accurately controlled by the tuning voltage input to the network.
Temperature drift of the bias regulation network can be avoided by incorporation of the network into a common heated package with the VCO.
The present invention thus prevents the problems of spectral breakup. The bias voltage is supplied to the varactor through an emitter follower circuit, and the reactances between the emitter follower circuit and the varactor itself are kept as small as possible. The bias voltage of the varactor is thus actively stabilized, so that the rectified RF current cannot dynamically change the DC voltage across the varactor. Parametric oscillation modes are thus suppressed, and spectral breakup is avoided.
The circuit of the present invention has the further advantage of limiting the current as breakdown voltage of the varactor is approached, so that the device itself is better protected.
A further advantage of the present invention is that the bias voltage is not actively regulated at high bias voltages, but simply passed to the varactor through a resistive network. The problems of rectification of the RF signal are normally worst at the low bias values, so that active regulation at higher bias values is unnecessary, and its elimination avoids the possibility of other instabilities appearing.
The present invention is preferably embodied as an emitter follower circuit, but of course this is not necessary. The key teaching of the present invention is that
(1) The varactor bias should be regulated by a bias regulator circuit which has a response time corresponding to a frequency which is at least 10% of the minimum frequency of oscillation of the RF oscillating circuit of which the varactor is a part, and
(2) The impedance between the bias regulator circuit and the varactor must be kept as low as possible.
In the sample embodiment primarily discussed, the varactor bias regulator circuit is off-chip from the varactor RF oscillator. However, it is contemplated that in future preferred embodiments the varactor bias regulator circuit can be integrated with the RF oscillating circuit. This has the advantage of further reducing the inductance between the varactor bias regulator and the varactor itself, which is desirable. This interface is also thereby made more reliable and more controllable, since bond wire lead inductances are not required.
The emitter follower circuit of FIG. 1 is a sample embodiment of the invention, but other circuits can be used. It is necessary that the regulator circuit be a relatively high-frequency circuit, because of the speed with which parametric oscillation modes can develop in the RF circuit which the varactor is part of. Preferably the bias regulator circuit, including the inductive impedance intervening between the bias regulator circuit and the varactor itself, is able to respond at at least 10% of the highest RF frequency (within the operating range of the RF circuit of which the varactor is a part) at which the varactor generates a substantial rectified current component. More generally, the varactor bias regulator circuit preferably has a maximum response frequency which is at least 10% of the minimum frequency of oscillation of the RF circuit of which the varactor is a part.
Thus, the present invention provides major advantages of reliability, spectral purity, and uniformity and reproducibility over prior art varactor circuits.
The present invention is particularly advantageous in varactor-controlled voltage controlled oscillators which are used for local oscillators in microwave receiver circuits, or in other downconverter circuits. However, the present invention may also be advantageous in suppressing parametric oscillation modes in other microwave circuits which use varactors.
In the receiver application, the greater spectral purity of the varactor-controlled voltage-controlled oscillator (VCO) local oscillator of the present invention is particularly advantageous, since it means lower noise in the downconverted signal and greater tuning accuracy. These advantages in turn mean that following stages can be less complex.
The problem of RF rectification to produce dynamic self-biasing in varactors is particularly a problem when the varactor is a hyperabrupt varactor, and therefore the present invention is particularly advantageous when used in wide-band microwave voltage-controlled oscillators or wide-band microwave receivers.
Another way of regarding the advantages of the invention is that it permits use of varactors even at points within their range where a substantial rectified current is generated--i.e., for a given varactor device structure, the lower limit of the tuning range can be extended.
According to the present invention there is provided: A microwave varactor circuit comprising: a varactor; a microwave oscillating circuit connected to said varactor and having a minimum and a maximum microwave frequency of oscillation; a varactor bias regulator, connected to said varactor to supply a bias voltage thereto; said bias regulator having a response time corresponding to a frequency which is at least 10% of said minimum microwave frequency of oscillation; said bias regulator being connected to said varactor with an intervening impedence which is less than 10 ohms at said minimum microwave frequency of oscillation.